Wireless sensors are devices in which sensing electronic transducers are separated from their associated readout/display components. Wireless sensors had been applied in temperature, pressure, and pH value. It also can be found in monitoring of many gases such as volatile organic compounds, toxic industrial chemicals, and chemical warfare agents in relatively interference-free industrial and indoor environments.
However, in these practical gas sensing applications, the available wireless gas sensors are not easy to meet the sensing requirements in complex environments.
To solve the fundamental selectivity and sensitivity issues, a combination of the three key sensor system components such as sensing material, transducer, and signal generation and processing techniques was utilized.
Until now, there are several battery-free passive wireless sensing technologies based on magneto-elastic, thickness shear mode, surface acoustic wave, magnetic acoustic resonance, and resonant LCR (inductor-capacitor-resistor) transducers.
Except for LCR transducers (RFID sensor tags), most of wireless sensing technologies request extra sensor IC circuits. RFID systems have been widely used in many applications ranging from logistics, to goods tracking, access control, automatic identification of animals, and so on.
With reference to FIG. 1, low frequency RFID systems (125-134 kHZ) and high frequency RFID systems (13.56 MHZ) have short transmission ranges from several centimeter up to 1 meter. Ultra-high frequency (UHF) RFID systems (860-960 MHZ) can cover a range up to 10 meters. Microwave frequency RFID systems (2.4 GHZ) cover transmission ranges up to 30 meters. For long distance sensing in security management and control system, only UHF and microwave RFID systems can meet the requirement.
Referring to FIG. 2, methods and systems for integrated interrogation of RFID sensors were disclosed in U.S. Pat. No. 8,717,146. To achieve accurate and precise sensing, several calculated spectral parameters, included the frequency position Fp and magnitude Zp of Zre(f) and the resonant F1 and anti-resonant F2 frequencies of Zim(f), were measured. When interest gas was adsorbed on surface of RFID sensor, dielectric constant of sensing film will change, which results in a shift on impendence parameters of RFID sensor antenna. According to this mechanism, LCR transducer (RFID sensor tag) becomes a promising wireless sensing technology.
An embodiment of a process for the fabrication of an RFID sensor has been illustrated. They also provided methods and systems for calibration of RFID sensors used in manufacturing and monitoring systems as disclosed in U.S. Pat. No. 7,911,345. For example, they proposed a selected sensing material applied onto the RFID antenna that altered its impedance response when interest gas was adsorbed on surface of sensing materials. A complementary sensor resistor and/or capacitor was also attached across an antenna and an IC memory chip to calibrate the sensor impedance response.
A sensing material layer directly coated on surface of HF and UHF RFID antenna has been disclosed in U.S. Publication No. 20140095102. In this design, one selected sensing material was coated on one tag, which is limited to only detect one target species by one tag.
For homeland security applications, long distance RFID sensor for detection of explosives is requested. So HF RFID sensor cannot be used in such long distance application.
Due to low vapor pressure of nitro explosive gases, how to improve the sensitivity, selectivity, reading range of present RF sensors is important issue for realizing it on homeland security applications. Requirements of explosive gas sensor contains be extremely sensitive, be highly selective, and be robust & stable.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.